1. Field of the Invention
The present invention relates to a strained quantum well laser that emits at high output, and has a low temperature dependence of efficiency and threshold current. More specifically, the present invention relates to a compressively strained quantum well semiconductor laser employing GaInAs/GaInAsP, in which a carrier blocking layer has been formed.
This application is based on Japanese Patent Application No. Hei 9-259124 filed in Japan, the content of which is incorporated herein by reference.
2. Discussion of the Background
GaInAs/GaInAsP is one of the most important semiconductor laser materials. In particular, an InGaAs/InGaAsP semiconductor laser having quantum wells in the active layer has been put into practical use as a communications light source in the wavelength band from 1.3 xcexcm to 1.55 xcexcm. In order to emit at wavelengths of 1.6 xcexcm or higher employing this material, a compressively strained quantum well semiconductor laser is known in which the active layer is the compressively strained layer.
FIG. 4 shows the band structure in a conventional InGaAs/InGaAsP compressively strained quantum well semiconductor laser for the case where there are three quantum well layers. FIG. 5 shows the multiple layer in this semiconductor laser.
In the design of this conventional semiconductor laser, a p-type cladding layer 2 (p-InP) is layered onto a substrate consisting of p-type InP. Next, sequentially layered on top of p-type cladding layer 2 are a p-type waveguide layer 3 (p-GaInAsP); GaxIn1xe2x88x92xAs first quantum well layer 4; GaInAsP barrier layer 5; GaxIn1xe2x88x92xAs second quantum well layer 6; GaInAsP barrier layer 7; GaxIn1xe2x88x92xAs third quantum well layer 8; n-GaInAsP waveguide layer 9; and an n-type cladding layer 10 (n-InP). The portion from GaxIn1xe2x88x92xAs first quantum well layer 4 through GaxIn1xe2x88x92xAs third quantum well layer 8 forms active layer 20.
In order to expand laser applications in medical instruments, manufacturing, and spectroscopy, as well as in optical sources for fiber communications, it is desirable that the above-described InGaA/InGaAsP semiconductor laser emit at high output and have a low temperature dependence of threshold current and efficiency.
For example, laser spectroscopy is an analysis method for measuring the gas concentration by irradiating the gas to be analyzed with semiconductor laser light of a specific wavelength band, and then detecting the amount of absorption by the specific gas components in the analyzed gas at a light receiving member. When sweeping the emission wavelength by varying the holding temperature of the semiconductor laser light source, the threshold current and slope efficiency are extremely important causative factors. Namely, the lower the holding temperature, the higher the slope efficiency and the smaller the threshold current. At the same time, the loss coefficient increases as the temperature rises. As a result, the external quantum efficiency drops and output falls. For this reason, it is desirable that the light source of the spectrometer have a low temperature dependence within the temperature range of 20xcx9c50xc2x0 C., without a change in the slope efficiency and threshold current.
The phenomenon of carrier overflow may be cited as the primary reason for deterioration in the temperature characteristics of a semiconductor laser and hindrance to achieving a higher output. In carrier overflow, recombination of electrons and holes injected into the active layer region cannot keep pace when an extremely high current is injected. As a result, electrons overflow into the region of waveguide layers 3,9 which has a larger band gap than active layer 20. The majority of electrons which overflow due to this type of phenomenon are consumed uselessly as heat loss. In addition, due to a rise in component temperature, carrier leakage increases exponentially. As a result, laser output cannot be increased and injection efficiency falls.
A Separated Confinement Heterostructure (SCH) employing quantum wells such as shown in FIG. 4 has been employed in order to improve the circumstances described above and prevent carrier overflow. Namely, a weak waveguide laser structure having a thin active layer has been investigated, with a focus on separating and confining light and carriers, or on reducing the laser power density. In addition, in order to improve the characteristic temperature, investigations have also been made into the use of compounds including Al in the active layer, and of a Grated Index Separated Confinement Heterostructure (GRIN structure), and the like.
However, when oscillating an InGaAs/InGaAsP semiconductor laser at a wavelength of 1.6 xcexcm or higher, the characteristic temperature (temperature dependent coefficient for threshold value) is at best 60K.
On the other hand, in case of emitting a semiconductor laser at high power, a method is known for introducing a carrier blocking layer into the active layer so that carrier overflow upon high injection of the carrier is prevented. For example, Japanese Patent Application, First Publication No. Hei 7-23139 discloses a semiconductor laser which emits at xcx9c1 xcexcm employing AlGaAs in which a carrier blocking layer has been introduced.
However, it is thought that the characteristics of the semiconductor laser may be effected by introduction of a carrier blocking layer, depending on the thickness of the layer. In other words, if the carrier blocking layer is made thin, on the order of 1xcx9c2 nm, the injected electrons are expected to easily transmit through the carrier blocking layer. Accordingly, an improvement in carrier overflow from this type of design cannot be readily anticipated. This is because, an electron tunneling effect may occur in the case of a thin blocking layer. If the blocking layer is extremely thin, the effect from the tunneling effect is extremely large. As a result, function as a blocking layer deteriorates. However, when the blocking layer is extremely thick the injection efficiency becomes very poor. This leads to an increase in the threshold current as a result, so that continuous emission at room temperature may no longer be possible. Based on these circumstances, then, it is believed that there is an optimal value for the thickness of the carrier blocking layer.
An evaluation based on temperature characteristics is performed as one indicator which prevents the carrier overflow phenomenon. In particular, there has been strong demand for improvement in temperature characteristics in InP optical devices.
As discussed above, there has been a desire in the various fields mentioned for a technique for further improving the temperature dependence of threshold current and efficiency in semiconductor lasers.
The present invention has as its objective the provision of a semiconductor laser which can emit at high output, for which the threshold current and efficiency have a low temperature dependence.
The present invention""s semiconductor laser is characterized in that, in a compressively strained quantum well semiconductor laser employing GaInAs/GaInAsP, carrier blocking layers are provided in the p-type and n-type waveguide layers having a band gap Egc which is larger than the band gap Egb of the waveguide layers, these carrier blocking layers being formed of a material having a refractive index which is smaller than the waveguide layer.
The thickness of the carrier blocking layers is preferably 5xcx9c10 nm.
In the case of a 1.74 xcexcm semiconductor laser, it is preferable that the band gap difference between the waveguide layer and the carrier blocking layer at room temperature be in the range of 85 meVxcx9c190 meV.
It is also preferable that the band gap difference between the band gap energy (Ego) of the fist ground state level in the quantum well layer and the band gap (Egc) of the carrier blocking layer be in the range of 300xcx9c500 meV.